Fig. 1: Stacked X-ray images of the emission around the central galaxies of rich galaxy clusters (left) and lower mass galaxy groups (right). These are two of the twenty produced in this study. In both images, the black circle indicates the radius "R500", which roughly matches the size of the dark matter halo. The X-ray emission is centrally concentrated but clearly extends out to a significant fraction of this radius. The numbers in the top right of each image denote the stellar mass of the central galaxies (log M_star; see Fig.2) which were stacked. As the rulers show, R500 is about 2.5 times larger (and the mass about 15 times larger) for the clusters than for the groups. However the radial distribution of emission is similar in the two images.
Fig. 2: Average X-ray luminosity for each of the 20 stacked images as a function of the stellar mass of the central galaxy. At higher masses the relation between the two is a power law (a straight line in this plot). For the seven data points at lowest mass, the X-ray emission from the hot gas is too faint to measure reliably, and the X-ray signal is also contaminated by emission from X-ray binaries in these galaxies - their estimated luminosity is shown with the blue and red dotted lines, corresponding to high-mass X-ray binaries and low-mass X-ray binaries respectively.
By combining data for more than 250,000 individual objects, an MPA-based team has for the first time been able to measure X-ray emission in a uniform manner for objects with masses ranging from that of the Milky Way up to that of rich galaxy clusters. The results are surprisingly simple and give insight into how ordinary matter is distributed in today's universe, and how this distribution has been affected by energy input from galactic nuclei.
Mike Anderson, Massimo Gaspari, Simon White (MPA), Wenting Wang (Institute for Computational Cosmology, University of Durham), Xinyu Dai (Department of Physics and Astronomy, University of Oklahoma)